The present invention relates generally to a laser range finder, and more particularly to a method and a device for enhancing the precision without increasing sampling frequencies and economizing the use of the memory of the laser range finder.
The conventional laser range finder emits the laser pulse in the range of 10 ns to 20 ns at an object which reflects back a laser signal. The laser signal is received by a laser receiver. The range is calculated by a formula as follows:
Td=2L/Cxe2x80x83xe2x80x83(1) 
The letter xe2x80x9cLxe2x80x9d of the formula (1) stands for the distance between the laser range finder and the object; xe2x80x9cCxe2x80x9d, speed of light; xe2x80x9cTdxe2x80x9d, time difference between the emitted laser pulse and the received laser pulse. The range can be calculated with precision by measuring Td on the basis of the above formula (1). The precision measurement of Td can be attained by enhancing the laser emitting power, or by eliminating the noise signal which is brought about by the sunlight and is received by the receiver. The U.S. Pat. No. 3,644,740 discloses a receiving circuit for obtaining an error warning by controlling the circuit bias of the receiving circuit, so as to improve the signal-noise ratio of the receiving circuit. The U.S. Pat. No. 4,569,599 discloses a counting control technique for detecting a range signal. The U.S. Pat. No. 4,770,526 discloses a technique for increasing the range detection value by amplifying the time delay signal. The U.S. Pat. No. 3,959,641 discloses a digital measurement technique for reducing the critical voltage of the laser receiver so as to enhance the range measurement. The U.S. Pat. No. 5,612,779 discloses a technique to adjust a threshold voltage automatically such that the threshold voltage is variable in response to the strength of the reflection signal of an object. As a result, a threshold voltage can be set between the noise signal and the object signal under various circumstances. Such prior art techniques as described above are intended to increase the range of measurement as well as the precision of measurement.
The conventional way for enhancing the reception capability and the measurement precision of a laser range finder involves the use of the high speed sampling statistical method and an increase in the sampling frequency, thereby enabling the sampling time point to be close to the time point of the actual reception of the laser signal. The precision range calculation is attained on the basis of the time point data. However, the sampling method is not cost-effective in view of the fact that a memory with a large storage capacity is needed for recording a number of the sampling reference points. Once more sampling frequency, then the more memory is needed. In addition, the high sampling frequency must be done by a high-speed element which consumes a great deal of electricity and costs expensively.
The primary objective of the present invention is to provide a method and a device for enhancing the precision of a laser range finder without an increase in the sampling frequency between the identical range finding blocks.
It is another objective of the present invention to provide a method and a device for reducing the need of large storage capacity of a memory by dividing a plurality of measurement blocks between the identical range finding blocks.
The method of the present invention includes a first step in which the measurement block number and the laser emission time are set up. Thereafter, the sampling of the laser reflection signal of each block is done in sequence, beginning from the first block, so as to determine a block in which an object is located. A laser beam of a predetermined delay time is emitted to the block in which the object is located. The reflective laser signals are sampled and processed to calculate a precise distance between the laser range finder and the object.
The device of the present invention comprises a timing interval generator, a microprocessor unit, a programmable delay unit, a laser transmitter, a laser receiver, a high gain amplifier circuit and a memory. The programmable delay unit and the microprocessor unit are used to control the time of the delay emission of the laser signal, thereby resulting in the time difference of the relative range signals.